1. Field of the Invention
This invention relates to electrochemical cells employing an alkali metal and a solid electrolyte.
2. Prior Art
A typical example of such a cell is a sodium sulphur cell. In such a cell liquid sodium constitutes the anode and is separated by a solid electrolyte, usually a beta alumina ceramic material, from a cathodic reactant comprising liquid sulphur and sodium polysulphides. In order to maintain a high rate of charge and discharge, the anode surface of the electrolyte has to be kept covered with liquid sodium. Sodium passes through the electrolyte on discharge of the cell and it is known therefore to provide capillary or other wicking means to maintain the electrolyte surface wetted with sodium which is drawn from a reservoir. It is also known to make use of gas pressure in a sodium reservoir to force the liquid sodium into the region adjacent the electrolyte surface (see for example U.S. Pat. No. 3,953,227). Such a gas pressurised cell requires the filling of the sodium compartment in such a manner that the gas pressure is sufficient even when the sodium in the anodic region is depleted on discharge of the cell, to force liquid sodium upwardly over the electrolyte surface. This raises considerable problems in the manufacture of such cells. In the specification of British Patent Application No. 41680/75 (U.S. Application Ser. No. 730,711, now U.S. Pat. No. 4,104,448) there is described and claimed an alkali metal-sulphur cell of tubular form in which the anode compartment is divided into first and second gas-tight chambers, the first chamber defining an annular space around the electrolyte tube and communicating with the second chamber to enable the passage of liquid alkali metal between the chambers, gas spaces being created above the alkali metal in the two chambers by porous masses which are not wetted by the liquid alkali metal, volumes of said spaces being such that, at the operating temperature of the cell, the gas pressure in the second chamber is higher than the gas pressure in the first chamber, whereby liquid alkali metal is forced from the second chamber into the first chamber to maintain the operative surface area of the electrolyte tube covered with liquid alkali metal during discharge of the cell. More particularly the porous masses may comprise carbon felt or alumina felt. In the manufacture of such a cell, hot liquid sodium is filled into the anode compartment, the filling operation being carried out in an inert gas atmosphere so that the porous masses become filled with the inert gas, which may for example be nitrogen or helium or argon. The porous mass has a low heat conductivity and will remain relatively cool during the filling operation. After the cell has been sealed, on heating the cell to its operating temperature which is typically 350.degree. C., the gas in the porous material will have a much higher pressure and hence provides the required pressurisation on the liquid alkali metal to force this metal upwardly over the surface of the electrolyte. To achieve this objective the cells described in the above-mentioned specification have to have a porous gas-containing material which is not wetted by the liquid alkali metal and which serves to hold the gas in the alkali metal reservoir whilst the reservoir is being filled so as to ensure that there is gas retained within the reservoir.